1. Field of the Invention
The present invention pertains to an apparatus and method for improving the performance of coating devices adapted, particularly a coating device that applies a multi-layer film to an envelope of a light source. More particularly, the present invention relates to a radio frequency (RF) magnetron sputtering device and method which is capable of depositing a relatively thick film having a low film stress onto an envelope thereby increasing lumen gain.
2. Discussion of the Art
Thin film optical coatings, also known as interference filters, comprise alternating layers of two or more materials of different indices of refraction. Such coatings or films are well known and used to selectively reflect or transmit light radiation from various portions of the electromagnetic radiation spectrum, such as ultraviolet, visible and infrared radiation. The thin film optical coatings are used in the lamp industry to coat reflectors and lamp envelopes. One application in which the thin film optical coatings are useful is to improve the illumination efficiency, or efficacy, of lamps by reflecting infrared energy emitted by a filament, or arc, toward the filament or arc while transmitting visible light of the electromagnetic spectrum emitted by the light source. This decreases the amount of electrical energy required to be supplied to the light source to maintain its operating temperature.
The film generally comprises two different types of alternating layers, one having a low refractive index and the other having a high refractive index. SiO2 is a commonly used low index material while TiO2, Ta2O2, Nb2O5, ZrO2, and HfO2 are excellent candidates for the high refractive index material. With these two materials, an optical thin film filter, which is deposited on the outer surface of the lamp envelope, can be designed. The filter transmits the light in the visible spectrum region (3800 to 7800 angstroms) emitted from the light source while it reflects the infrared light (7800 to 25000 angstroms). The returned infrared light heats the light source during lamp operation and, as a result, the lumen output of a coated lamp is considerably greater than the lumen output of an uncoated lamp.
Known methods for applying interference filters or coatings to a light source include low pressure chemical vapor deposition (LPCVD) and sputtering techniques. While capable of producing a satisfactory interference filter for light sources, such techniques have limitations with respect to the uniformity of the coating, layer thickness, and packing density.
In a conventional LPCVD process, two reactive gases are introduced into a vacuum chamber where they chemically react to form a thin film over a substrate, such as a quartz or glass envelope. This process can be repeated several times to apply multiple layers on the substrate. It is well known in the art of coating light source envelopes that the greater the film stack or number of layers comprising the interference filter, the greater the lumen gain. However, the thickness limit for a lamp interference layer produced by LPCVD is approximately four micrometers (about 40 layers).
Interference filters greater than about four micrometers or 40 layers cannot be prepared for light sources by the process of LPCVD because the intrinsic stress in such a thick layer of film is too large. Relatively thick films will crack during thermal cycling, thereby forming a crack network across the lamp, that for thick films usually tears the quartz substrate and could lead to lamp failure. Even if the film cracking does not rupture, the cracked film surface has high scatter loss which reduces the application of the lamp. For these reasons, the thickness of lamp interference filters prepared by LPCVD is limited to a thickness of about four micrometers or 40 layers. The corresponding lumen gain (gain over uncoated lamp) for such a thickness is only about 38%. It is an object of the present invention to achieve significantly higher lumen gains.
As mentioned, sputtering techniques are also used to coat light sources. A typical sputtering device includes a chamber housing at least one target and a substrate. A gas, such as argon, is introduced into the chamber which becomes positively ionized. The positive argon ions bombard the target causing deposition material to be removed and condense into a thin film on the substrate. Sputtering techniques suffer from a number of shortcomings when used to prepare interference filters for light sources.
First, light sources are typically designed having elliptical envelope shapes. Thus, the target atoms sputtered from the target deposit on the lamp surface from different incident angles. As a result, the film deposited on the lamp surface is not uniform which negatively effects lumen output. Second, it is difficult to couple RF power to a quartz or glass lamp due to the low impedance of the lamp. By coupling RF power to the light source, one can produce a substrate bias which results in a denser, better performing film. Third, the high refractive index layer is often made from titanium dioxide (TiO2), which at temperatures of about 800 to 900xc2x0 C. changes to a crystal phase. Such a form of TiO2 is undesirable.
Thus, a need exists to provide a coating technique which overcomes the foregoing problems associated with LPCVD and known sputtering techniques.
A new and improved apparatus and method is provided for coating light sources which meets the foregoing needs.
In an exemplary embodiment a radio frequency (RF) magnetron sputtering device for lighting applications includes a magnetron sputtering chamber. First and second targets are housed by the chamber and are disposed on opposite sides of the chamber. The first target is made from a material having a relatively low refractive index and the second target is made from a material having a relatively high refractive index. Also enclosed by the chamber is a substrate carrying assembly. The substrate carrying assembly is adapted to carry a series of substrates back and forth between a position adjacent the first target where the substrate is coated with the material having the relatively low refractive index and a position adjacent the second target where the substrate is coated with the material having the relatively high refractive index.
The first and second targets are preferably disposed vertically within the sputtering chamber so that a symmetrical axis of each of the substrates lies in a horizontal plane.
An exemplary method for applying an interference layer to light sources using a magnetron sputtering technique includes the steps of providing first and second targets within a magnetron sputtering chamber on opposite sides of the sputtering chamber; providing a substrate carrying assembly within the sputtering chamber between the first and second targets; mounting a series of substrates to the substrate carrying assembly; and selectively bombarding the first and second targets with a plurality of positively charged ions so that a plurality of atoms sputter off the first and second targets and deposit on a surface of the substrates.
One advantage of the present invention is the provision of a coating device and coating method for lighting applications capable of applying relatively thick films having a low film stress to substrates.
Another advantage of the present invention is the provision of a coating device and coating method for lighting applications which reduces film scatter while increasing lumen gain.
Another advantage of the present invention is the provision of a coating device and coating method for lighting applications having enhanced variation control and environmental protection.
Another advantage of the present invention is the provision of a coating device and coating method for lighting applications capable of coupling a radio frequency power source to a series of light sources via cooper shots.
Another advantage of the present invention is the provision of a film layer having a high refractive index which does not experience phase changes at elevated temperatures.
Still another advantage of the present invention is the provision of a coating device and coating method for lighting applications capable of applying a uniform film having a thickness error less than 10% across the surface of a substrate.
Still another advantage of the present invention is the provision of a coating device and coating method for lighting applications capable of applying a film having relatively low absorption properties.
Yet another advantage of the present invention is the provision of a coating device and coating method for lighting applications capable of applying a dense film across the surface of a substrate.